Unfortunately my iMac is nowhere near the router so I can't use an ethernet cable. All the guides I've read say that due to the Mac’s limitation you can’t be connected to a Wi-Fi network and host a Wi-Fi network at the same time. But has anyone figured out a workaround?
I don't know about trying to extend a wireless link, wirelessly through a VPN - but you can certainly connect with ethernet from a longer distance than you can connect through wifi. All it takes is enough length for your ethernet cable. I have often read that 300 feet on ethernet is a good limit on un-managed networks, such as in a home. https://www.amazon.com/UbiGear-4445608-Ethernet-Network-Patch/dp/B00BNL001Y/
I think it's unlikely that you are that far from your wifi router.
But, then, that length of wired connection might require you to have a really-long wire laid out to make the connection, and that may not be practical (or neat, or safe) for some locations.
Just something else you can consider while you are trying things out.